Issue 16551: Cleanup the pure Python pickle implementation

This issue inspired by issue12848. The proposed patch get rid of the marshal module (the struct module used instead), removes some redundant code, and changes the code to use more modern idioms.

Ping.

Patch looks fine to me. Do you want to go ahead?

In response to Alexandre's comment on Rietveld. Access to a local variable is faster than to a global one and the current implementation uses this for struct.pack. I just use same trick for struct.unpack. Here is a microbenchmark which demonstrate some effect of this optimization. I got 0.6491418619989417, 0.6947214259998873, and 0.5394902769985492 for optimized, non-optimized and advanced optimized functions.

Of course, we can achieve even better effect if we will cache not only struct.pack, but struct.Struct('<i').pack, struct.Struct('B').pack, etc. I were considered this as a reason for other patch, but we can do it in this issue.

My point is I would prefer that we keep all optimizations to only the _pickle C module and keep the Python implementation as simple as possible.

Also, I doubt the slight speedup shown by your microbenchmark will actually result in any significant benefits on the overall performance of pickle.py. Furthermore, we can't use CPython to measure the benefits of such optimization because CPython will always use _pickle over the Python implementation.

Here is an updated patch which get rid of trick with attribute caching. Globals used instead. Also added some minor style changes (dropped redundant semicolons and wrapped too long lines).

Patch updated in response to Alexandre's comments. In additional to his suggestions some other minor things simplified. _batch_appends and _batch_setitems now use islice instead range. Some bugs found and new issues created (issue17710, issue17711).

New changeset 3dff836cedef by Serhiy Storchaka in branch 'default':
Closes #16551. Cleanup pickle.py.
http://hg.python.org/cpython/rev/3dff836cedef

nosy: + python-dev
messages: + msg186906

resolution: fixed
stage: patch review -> resolved

messages: + msg186716

messages: + msg186304

messages: + msg186205